Enter Three Murderers: Was Macbeth the third murderer?

From The tragedy of Macbeth according to the first folio. Ed. Allan Park Paton.

The question here starts up: Was Macbeth himself the third murderer at Banquo's death?

We do not remember having seen this suggested by any
Shakesperean commentator. Yet we think there are grounds for
believing that it was part of Shakespeare's design -- that he purposely
left it untold in words, and, as it were, a secret to be found out; and,
to any one accepting such a view, the tragedy will be found, we believe, deepened in effect.

The following are the circumstances on which we rest the
opinion:

1. Although the banquet was to commence at seven, Macbeth (as he had foretold his queen and courtiers) did not go there till near midnight.

2. He had no more than entered the room of state when the first murderer came to tell him of the deed, apparently freshly committed.

3. Absent and alone four or five hours, how had Macbeth been employed? With such a dreadful matter at issue, he could not have been resting or engaged in any other business. He must have been
taken up with the intended murder some way or other; and, for
ourselves, we cannot conceive of his going to the banquet with the
barest chance of his plot miscarrying, and of Banquo's arriving in
the midst of the gaiety, with the narrative of the inexplicable and
alarming attempt. But if he waited away till his mind would be
relieved by a knowledge of the assassination, this could not have
been, unless he was personally engaged in it, because it was after he
went that he was told. He had indeed actually commenced, in a
hearty and confident manner, his duties as host when the stained
messenger entered.

4. The two murderers employed (opposite types of evil instruments -- the one world-sick, and the other world-hating) Macbeth had been, as we know, at great pains to influence for his purpose; and if there had been a third man in whose hands he could have put himself, and to whom he could have committed the superintendence of the others, we certainly should have heard of that man. He would have been Macbeth's chief confidant, and as such would in all probability have been first to reach the banquet room, carrying the longed-for tidings.

5. The first murderer told Macbeth that he "cut Banquo's throat," that was his work; but there were twenty wounds in the victim's head -- "twenty mortal murthers." A needless and devilish kind of mutilation, not like the work of hirelings.

6. When the third murderer unexpectedly joined the others (be
it observed, just before the attack, as if he separately had been
listening for the returning travellers), he repeated the orders they
had got, so precisely as at once to remove their doubt. He was the
first to hear the sound of horse. He showed unusual intimacy with
the locality, and the habits of the visitors, &c. It was he who
identified Banquo. Probably to do away with the chance of his
being recognised, he seems to have struck down the light (although
he asked about it); and it was he who, searching the ground, found Fleance escaped.

7. Just as Banquo was struck down, the First Murderer, in his excitement, says:

"Well, let's away, and say how much is done."

But we know that he did not go at once, for at the feast he tells
Macbeth, that "safe in a ditch he bides." Some one must have reminded him that the body must be disposed of: it was not to lie about proclaiming murder; it was to be engulphed in a moorland rut.
Macbeth's words, after the appearance of Banquo's ghost, are:

"If Charnel houses, and our Graves must send
Those that we bury back; our monuments
shall be the maws of kites."

This "burying" allowed Macbeth to be back at the Castle, to get cleaned and robed, and be present at the feast. ("There's blood upon thy face," would apply to dried blood.)

8. There was a levity in Macbeth's manner in his interview with
the first murderer at the banquet, which has been frequently
remarked on by editors, &c., and which well might be if he
personally knew that Banquo was dead. (The passages, "Then
comes my fit again," &c., and "There the grown Serpent lies," &c., should, doubtless, be spoken to himself).

9. When the Spirit appears, Macbeth asks those about him "which of them had done it," evidently to take their suspicion off himself (for he knew); and his words --

"Thou canst not say I did it: never shake
Thy gory locks at me," --

sound very like "In yon black struggle you could never know me." We said that, accepting such a view, we thought the tragedy deepened in effect. For instance, it shows Macbeth's terrible
degradation in that he could personally, and along with hired murderers, assassinate his friend and fellow-soldier. The "twenty mortal murthers" exhibit the fear of criminal ambition in its utmost activity. In the king disguised, being but a little ago a murderer in the gloom, and now in his regal robes presiding over a banquet, we
have a striking contrast. And the shock he sustains on beholding Banquo's phantom is surely intensified through his certainty of his having himself destroyed him, and left him dead beyond all question.